Vehicle Speed

PRINCIPLE OF OPERATION, VEHICLE SPEED





Vehicle Speed--The speedometer sends a square-wave speed signal to pin 39 of the Trionic control module. The signal varies between 0 V and battery positive. At 30 km/h its frequency is about 20 Hz.


This signal is used for:

- Determining which gear is currently engaged
- Activating the SHIFT UP lamp (certain markets)
- Limiting boost pressure in 1st gear (all turbo versions except B204S and B234E)
- Activating fuel shut-off in 3rd, 4th and 5th gear (normally aspirated engines)
- Activating speed limiting (by limiting boost pressure at maximum speed).

If the speed signal is absent, the above functions are not performed and boost pressure is limited to the lowest gear-dependent pressure.